1. Field of the Invention
The present invention relates to a secondary air supply device configured to supply secondary air to an exhaust gas passage of an internal combustion engine provided with exhaust gas purifying catalyst, and more particularly to a secondary air supply device for an internal combustion engine including a means for calculating a flow amount of the secondary air.
2. Description of Related Art
There has been known that the internal combustion engine is provided with an exhaust air control apparatus in which catalyst having oxidative function is arranged in an exhaust gas passage thereof to reduce carbon monoxide (hereinafter referred to as “CO”), hydrocarbon (hereinafter referred to as “HC”) and nitrogen oxide in the exhaust gas (hereinafter referred to as “NOx”) to achieve purification of the exhaust gas. Further, there has been known also another secondary air supply device in which, by feeding air with pressure from an air pump to a secondary air supply passage having an on-off valve connected to the exhaust gas passage, the secondary air is supplied into the exhaust gas pipe to intensify the concentration of oxygen, so that HC and CO in the exhaust gas are oxidized to accelerate the purification of the exhaust gas.
If the internal combustion engine is used for a long term, minute particles mixed in intake gas taken from an intake system of the internal combustion engine adheres to an inside wall and the like of the secondary air supply passage so that they are accumulated as deposit. In addition, minute particles in the exhaust gas may also adhere to the inside wall and the like of the secondary air supply passage so that they may be accumulated as deposit. If the accumulation of deposit occurs in the secondary air supply passage, pipe resistance of the secondary air supply passage increases, so that pressure of the secondary air supply passage indicates a different behavior from when no deposit is accumulated within the secondary air supply passage. Then, when calculating the flow amount of the secondary air by means of a pressure sensor provided in the secondary air supply passage, there is a possibility that the flow amount of the secondary air may be determined to have increased by mistake although actually the flow amount of the secondary air has decreased. This sometimes may affect emission of the exhaust gas in the internal combustion engine. Thus, even if any deposit is accumulated within the secondary air supply passage, the secondary air supply device has been demanded to grasp the actual flow amount of the secondary air supplied to the exhaust gas passage by the secondary air supply device with a high accuracy.
Japanese Patent Application Publication No. 2005-163709 (JP 2005-163709 A) has disclosed a secondary air supply device which includes a pressure sensor in the secondary air supply passage between an air pump (AP) and an air switching valve (ASV). By calculating the flow amount of the secondary air based on a pressure detected by the pressure sensor when the air switching valve is opened while the air pump is driven and a pressure detected by the pressure sensor when the air switching valve is closed while the air pump is driven, even if any deposit is accumulated within the secondary air supply passage, the actual flow amount of the secondary air supplied to the exhaust air passage by the secondary air supply device is grasped.